The present invention relates to methods of depositing alternate layers of cadmium telluride and mercury telluride in the manufacture of an electronic device, particularly but not exclusively of cadmium mercury telluride. Such electronic devices may be, for example, infrared detectors or phototransistors.
Published United Kingdom patent application GB-A-2 146 663 discloses, in the manufacture of an electronic device such as an infrared detector of cadmium mercury telluride, a method of depositing alternate layers of cadmium telluride and mercury telluride on a substrate in a reaction zone inside a reactor vessel. The method comprises the steps of (a) passing a gas stream containing a volatile tellurium compound over the substrate while maintaning an atmosphere of mercury vapour in the reaction zone, and (b) switching on and off a supply of a volatile cadmium compound to the reaction zone so as to deposit cadmium telluride when the supply is on and to deposit mercury telluride when the supply is off. The whole contents of GB-A-2 146 663 are hereby incorporated as reference material into the present specification.
During the steps (a) and (b) in the method of GB-A-2 146 663, the substrate is heated to a temperature (preferably in the range 400.degree. to 430.degree. C., degrees Celsius) at which the cadmium telluride and mercury telluride interdiffuse during growth of the alternate layers on the substrate to form a single layer of cadmium mercury telluride. The composition (x) of the Cd.sub.x Hg.sub.1-x Te thus formed is determined by the relative thicknesses of the CdTe layer(s) and the HgTe layer(s). In principle, this method permits the growth conditions to be optimised for each CdTe and HgTe layer, while simply switching on and off the voltatile cadmium compound supply.
Published PCT international patent application WO-A-86/02951 discloses a modification of the method of GB-A-2 146 663 in which the surface of the substrate is uniformally irradiated with electromagnetic radiation (particularly ultra-violet light) during the deposition of all the CdTe and HgTe layers. This provides photolytic decomposition of the volatile Cd and Te compounds so that growth of the layers on the substrate can occur at lower substrate temperatures, for example below about 350.degree. C. The whole contents of WO-A-86/02951 are hereby incorporated as reference material in the present specification.
It is found that, at least when the Te and Cd compounds are alkyls and when using a carrier gas of hydrogen, the photodecomposition of the alkyls in this manner results in cadmium telluride being deposited as a dust of no practical use. The cause appears to be a strong vapour phase photolytic reaction of the cadmium alkyl (for example dimethyl cadmium) which is assisted by the hydrogen and which results in the homogeneous nucleation of CdTe particles. This problem is mitigated in the method of WO-A-86/02951 by using a carrier gas composed at least 50% (and preferably more than 95%) of an inert gas, for example helium. Furthermore, as the photodecomposition of dimethyl cadmium is much more efficient than that of diethyl telluride, the concentration of the cadmium alkyl in the gas stream is preferably much less than that of the tellurium alkyl.